Compositions for thermal energy storage or thermal energy generation

ABSTRACT

A composition for thermal energy storage or thermal energy generation comprising a silica based gel or dry powder in the form of silica particles containing a water/urea phase change material for thermal energy storage or an endothermic or exothermic compound for thermal energy generation. The water/urea phase change material stores and releases at least 50 cal/gm of thermal energy in freezing and melting, and has a melting and a freezing point in the range of -11° C. to -15° C. The endothermic compound is preferably ammonium nitrate, urea, or combinations thereof. The exothermic compound is preferably calcium oxide or calcium chloride. The thermal energy storage composition may find use in a variety of applications including medical wraps, food servingware, and &#34;blue ice&#34; for cold packs or food storage. The thermal energy generation composition may find use as medical wraps, food servingware, and refrigerators when endothermic and medical wraps, food servingware, heaters, stadium seats, boots, vests, caps ear muffs, and scarves when exothermic.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.08/228,147, filed Apr. 15, 1994, which issues as U.S. Pat. No.5,423,996, on Jun. 13, 1995.

BACKGROUND OF THE INVENTION

The present invention relates to compositions for thermal energy storageor thermal energy generation, and more particularly, to a compositioncomprising a silica based gel or dry powder containing a water/ureaphase change material for thermal energy storage or an endothermic orexothermic compound for thermal energy generation.

Phase change materials may be repeatedly converted between solid andliquid phases and utilize their latent heat of fusion to absorb, storeand release heat or cool during such phase conversions.

These latent heats of fusion are greater than the sensible heatcapacities of the materials. For example, in phase change materials, theamount of energy absorbed upon melting or released upon freezing is muchgreater than the amount of energy absorbed or released upon increasingor decreasing the temperature of the material over an increment of 10°C.

Upon melting and freezing, per unit weight, a phase change materialabsorbs and releases substantially more energy than a sensible heatstorage material that is heated or cooled over the same temperaturerange. In contrast to a sensible heat storage material that absorbs andreleases energy essentially uniformly over a broad temperature range, aphase change material absorbs and releases a large quantity of energy inthe vicinity of its melting/freezing point.

Ice/water phase change materials are low-cost, widely-used phase changematerials for temperature regulation at 0° C. Such phase changematerials have found use in applications such as refrigeration, chillingof beverages, medical therapy, and frozen confections. The manyapplications for ice/water phase change material could be significantlyincreased if a means could be found to decrease the freezing temperaturewithout a prohibitive concurrent decrease in thermal energy storage.Soluble additives such as salt, alcohol, glycol, glycerine, or sugar,all function to depress the freezing point of water to temperatures wellbelow 0° C. but these additives also decrease the heat of fusion to 50%or less that of pure water. Further, such additives, when mixed withice/water, are messy and inconvenient to use.

If, for example, ice/water could be made to freeze and melt congruentlyat a temperature in the range of -11° to -15° C., without a substantialdecrease in the heat of fusion and crystallization, many newapplications would become possible and current applications improved.The new modified ice/water could be used to freeze pure water, make icecream, keep cold drinks colder, store cool for off-peak electrical airconditioning, and thermal energy storage of "cool" for diurnal andseasonal cooling. Thus it could be used in a device for making frozenconfections such as that disclosed in Uesaka, U.S. Pat. No. 4,488,817.Uesaka discloses using a cold-keeping agent which comprises water orcarbonated water with organic or inorganic salts added thereto in adouble-walled vessel for that purpose.

Further, if there were a means for containing the new modified ice/waterso as to encapsulate it or to render it a dry powder, then, its usecould be greatly expanded beyond that discussed above. In my U.S. Pat.Nos. 5,106,520 and 5,282,994 there is disclosed a free flowing,conformable powder-like mix of silica particles and a phase changematerial which may include water. Still there is no disclosure of theuse of a water/urea clathrate or inclusion compound as the phase changematerial.

Thus, while phase change materials for thermal energy storage are known,improved thermal energy storage materials would be desirable. Likewise,improved endothermic and exothermic compounds for thermal energygeneration are also in demand.

Instant cold and instant hot products for medical therapy and other usesare known. There are a number of instant cold packs on the market basedon ammonium nitrate/water. These products usually contain a freezingpoint depressant (to prevent freezing to a hard ice in reuse); and, insome cases, a gelling agent as well to produce a somewhat reusable gel.One of the instant hot products commercially available is based onsupercooling of a salt hydrate, sodium acetate trihydrate, that isinitiated by mechanical attrition to start crystallization and supplyhot at about 50° C. The product can be reactivated by remelting inboiling water to provide limited reuse capability as a gel. With boththe instant cold and instant hot products, the reusable gel has only alimited effectiveness and a limited useful life. Accordingly, a gel witha superior reuse capability would be advantageous. Perhaps even moreadvantageous would be a dry powder containing an endothermic orexothermic compound because of its soft conformability. As discussedabove, dry powders containing phase change materials are disclosed in myU.S. Pat. Nos. 5,106,520 and 5,282,994, but there is no disclosure ofusing endothermic or exothermic compounds in that regard.

Accordingly, there is still a need in the art for improved compositionsuseful in thermal energy storage or thermal energy generation which areinexpensive and easy to use.

SUMMARY OF THE INVENTION

The present invention meets that need by providing a silica based gel ordry powder in the form of silica particles containing a water/urea phasechange material for thermal energy storage or an endothermic orexothermic compound for thermal energy generation. In one embodiment,there is provided a water/urea phase change material contained in aparticulate silica matrix. Preferably, the silica particles arehydrophobic silica particles surface treated with 0.5-5 parts perhundred by weight of a silane coupling agent or silicone resin. Themixture of silica and phase change material (PCM) is preferably in theform of a free-flowing, conformable powder-like mix, i.e. PCM/silica drypowder, which may be prepared in accordance with U.S. Pat. Nos.5,106,520 or 5,282,994, which are incorporated herein by reference. Thewater/urea PCM/silica dry powder preferably has a thermal energy storageof greater than 30 cal/gm.

In this embodiment, the silica is preferably present in an amount offrom 30 to 40% by weight and the water/urea phase change material ispresent in an amount of from 70 to 60% by weight. This type of structureis especially desirable for medical wrap applications, but is ofinterest in other applications such as for tableware. Thus, thePCM/silica dry powder may be disposed in a liquid impervious polymerfilm or a metal foil enclosure to form a medical wrap. The PCM/silicadry powder may also be disposed in the plastic housing of containerssuch as tableware items or ice cream freezers.

Likewise, the water/urea phase change material/silica dry powdercomposition may be disposed in the inner chamber of a housing where thehousing includes an inner cavity for containing a heat sensitive itemsuch as a flight recorder. Such a device is disclosed in copendingapplication Serial No. 08/044,819, incorporated herein by reference.

The water/urea phase change material is a water/urea clathrate orinclusion compound which melts and freezes congruently in the range ofabout -11° C. to -15° C., and stores and releases over at least 50cal/gram, preferably over 60 cal/gram, and up to 72 cal/gram of thermalenergy in melting and freezing. The water/urea phase change materialpreferably is less than about 80% by weight water, preferably within therange of about 82-54.5%, and more preferably within the range 78-70% andat least about 20% by weight urea, preferably within the range 18-44.5%and more preferably within the range 22-30%. In the most preferredembodiment, the water/urea phase change material is about 75% by weightwater and about 25% by weight urea.

The thermal energy storage composition of the present invention isuseful in a variety of applications. For example, the composition may beused in medical wraps, food servingware, and "blue ice" for cold packsor food storage.

The water/urea phase change material may also be used in neat form. Forexample, the water/urea phase change material by itself may be disposedin a bag or container made of a liquid impervious polymer for use as"blue ice" or it may be disposed in the plastic housing of containerssuch as tableware items or ice cream freezers.

In another embodiment, there is provided a silica based gel or drypowder composition capable of generating high endothermic cool orexothermic hot, when activated by a liquid activating solution, such aswater. When water, or another phase change material is used as theliquid activating agent, the composition is thereafter capable of beingreused as a thermal energy storage device. For a compound to supply highendothermic cooling or exothermic heating, the chemical must have a highnegative or positive heat of solution in a liquid activating solutionsuch as water, a relatively low molecular weight, combined with highsolubility in a liquid activating solution such as water (at or nearambient temperature). Additionally, the chemical must be non-toxic,environmentally safe, and available at reasonable cost.

For instant cooling, ammonium nitrate, which has a high molalendothermic heat of solution, low molecular weight, and high solubilityin water, is non-toxic and low-cost. Urea also has a relatively highendothermic heat of solution (second to ammonium nitrate), and isnon-toxic and is environmentally acceptable.

Accordingly, in the preferred embodiment for instant cold applicationsammonium nitrate, urea, and combinations of these two chemicals are usedwith hydrophilic fumed or precipitated silica particles, with andwithout a freezing point depressant, to produce a reusable gel at lowersilica concentrations (i.e. 20 to 30% by weight of the totalcomposition); and a soft, comformable dry powder at higher silicaconcentrations (i.e. 30 to 40% by weight of the total composition). Inthis system, salt can function as a freezing point depressant to providereuse capability in the gels, but is not essential in the dry powdercompositions that contain a high amount of silica. However, in the gelsor dry powder the urea can supply significant added thermal energystorage if it is added in solid powder form and mixed with the ammoniumnitrate prior to activating the system with water.

The gels will have reuse capability only as sensible heat materials,since a freezing point depressant is present to prevent freezing to ahard lump of "ice." The dry powder, on the other hand, can be reused asa phase change material if recharged by freezing, and without forming ahard lump of ice.

Most of the compositions having the highest exothermic heat of solutionand highest solubility present obvious problems of toxicity, thermalinstability, etc. The hydration of calcium oxide (lime) with waterproduces excessively high temperatures and forms a solid brick. Still,this vigorous exothermic reaction may be of interest as ahigh-temperature heat source for non-medical use. The preferredexothermic material for medical therapy applications is calciumchloride. Calcium chloride may, thus, be added to hydrophilic fumed orprecipitated silica particles to form a gel at lower concentrations ofsilica (i.e. 15 to 25% by weight of the total composition) and drypowders at higher concentrations of silica (i.e. 25 to 40% by weight ofthe total composition). If desired, added temperature exotherm may beobtained by a combination of one highly exothermic compound such ascalcium chloride and a second exothermic compound such as calcium oxide,potassium carbonate or others.

Suitable means for containing the separate dry and liquid activatingsolution ingredients must be provided for the instant cold and instanthot systems. Film containers in which the liquid activating solutionsuch as water is contained in a "rupturable" bag inside a larger bagcontaining the dry ingredients is one system that is described in theU.S. Pat. 3,874,504, which is incorporated herein by reference, andwhich may be used. However, to avoid the possibility of accidentalactivation, the water may be contained in a separate small bag attachedto the outside of the larger bag. To activate the system, the larger bagis opened and the water poured therein and rapidly mixed to produce aninstant hot gel or dry powder.

The primary use of the instant cold and instant hot gels and dry powdersis probably for medical therapy where they supply sensible energy on atime/temperature thermocline. In this use they may supplement thethermal energy storage material based on the water/urea phase changematerial of the first embodiment discussed above. The instant cold andinstant hot gels and dry powders can be activated anytime and anyplace,and have reuse capability. Additional applications for which the instantcold and instant hot gels and dry powders may be used include clothingand wearing apparel, home ice cream freezing, outdoor cooking, andseasonal thermal storage.

Accordingly, it is an object of the present invention to provide acomposition for thermal energy storage or thermal energy generationcomprising a silica based gel or dry powder containing a water/ureaphase change material for thermal energy storage or an endothermic orexothermic compound for thermal energy generation. These, and otherobjects and advantages of the present invention, will become apparentfrom the following detailed description and the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a medical wrap utilizing thecompositions of the invention;

FIG. 2 is a sectional view taken along the lines and arrows 2--2 shownin FIG. 1 wherein a water/urea phase change material/silica dry powdercomposition of one embodiment of the present invention is contained;

FIG. 2A is a magnified cut-away view showing the uniform nature of thewater/urea phase change material/silica dry powder;

FIGS. 3A-3D show an elevational view, broken away, of formation of apack for-thermal energy generation utilizing the endothermic orexothermic compound/silica dry powder composition of another embodimentof the present invention;

FIG. 4 is a diagrammatic view of a tableware item, a dinner servingtray, utilizing the compositions of the present invention; and

FIG. 5 is a sectional view taken along arrows 5--5 in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, FIGS. 1, 2 and 2A show a medical wrap 2,specifically an elbow joint wrap, comprising an outer envelope 4, formedfrom a liquid impervious material which may be a polymeric material suchas a butadiene-acrylonitrile copolymer, a polyester such as polyethyleneterephthalate or vinyl polymer such as plasticized polyvinyl chloride,plasticized polyvinylidene chloride, low and high density polyethyleneand ethylene-vinylacetate copolymers or a metal foil such as aluminumfoil. Housed within the liquid impervious outer envelope is apowder-like mix comprising a silica matrix 8 containing a phase changematerial 6. The PCM/silica dry powder is shown diagrammatically only. Inactual practice the phase change material is absorbed or adsorbed withinand throughout the porous structure of the silica matrix. In any eventthe medical wrap 2 may also comprise fastener means such as "Velcro"strips (not shown) to provide for attachment of the wrap around thedesired anatomical body part.

The preferred silica for use in the PCM/silica dry powder is ahydrophobic silica that has been surface treated with 0.5-5 pph (partsper hundred by weight) of a reacted silicone resin or a silane couplingagent such as dimethyldichlorosilane. As used herein in thespecification and claims, hydrophobic silica is used to refer to asilica wherein the surface hydroxyl groups normally present have beenreacted with silicone resins or silane coupling agents to form a lesspolar "hydrophobic" surface. The silica particles treated in such amanner may be either fumed silicas or precipitated silicas. Exemplarysilicas include precipitated silicas such as those disclosed in U.S.Pat. Nos. 5,106,520 and 5,282,994, incorporated herein by reference. Thepreferred silica particle size is from about 7×10⁻³ to about 7×10⁻²microns.

In FIGS. 3A-3D the liquid activating solution, such as water, foractivating the endothermic or exothermic activity of the composition forthermal energy generation fills a small, sealed, liquid-tight inner bag10 of suitable flexible plastic that is relatively easily ruptured bythe liquid when squeezed manually. Inner bag 10, filed with the liquidactivating solution is inserted into an open end of an open-toppedintermediate envelope 11 which may be made of a liquid imperviousmaterial as in FIGS. 1, 2 and 2A. The remainder of envelope 11 is thenfilled or substantially filled with composition 12 of the presentinvention, in this instance a silica based dry powder containing anendothermic or exothermic compound for thermal energy generation.Following that, the top of intermediate envelope 11 is sealed. When thesqueezing occurs, inner bag 10 is ruptured, enabling the liquidactivating solution to mix with the silica based dry powder containingthe endothermic or exothermic compound, causing the latter to absorb orrelease heat, depending on the type of material it is, but the activatedmixture remains sealed in intermediate envelope 11.

After being filled, as described, and before being squeezed to absorb orrelease heat, the sealed intermediate envelope 11 is slidably insertedinto an open-topped outer pouch 13, that is just slightly larger insize. Preferably, the outer pouch 13 is of flexible, transparentplastic, such as polyethylene or other impervious material as in FIGS.1, 2 and 2A, and it is relatively flat, with its opposite major facesjoined integrally to one another at the opposite side edges 14 and 15and sealed to each other along the bottom of edge 16.

A thin, flat, flexible sheet 17 of heat insulation material may beinserted into the outer pouch 13 either before or after the insertion ofthe sealed intermediate envelope 11. This heat insulation sheet may beof fine cell or cross-linked polyethylene or other suitable material,and it extends substantially completely across the inside of one majorface of the outer pouch 13 and separates this side of the pouch from thesealed intermediate envelope 11.

After both the insulation sheet 17 and the sealed inner envelope 11 havebeen inserted, the top of the outer pouch 13 is sealed to provide aliquid-tight package.

Preferably, the outer pouch 13 is of suitable flexible, transparentplastic and, in the absence of a defect, it does not rupture whensqueezed manually. The outer pouch 13 constitutes a means for affixingthe insulation sheet 17 to envelope 11. The affixing means could takeother forms such as adhesive, but the outer pouch is preferred andadvantageous. For example, the outer pouch provides extra protectionagainst leakage.

Before using this instant cold or instant hot pack to either heat orcool, the pack provides a relatively flat package containing the sealedinner envelope 11 in which the particles of composition 12 aresegregated from the liquid activating solution by the rupturablemembrane constituted by the inner bag 10.

After the thermal energy generation upon first use, the mixture inintermediate envelope 11 may serve as a thermal energy storage device,and by heating or cooling the pack, as the case may be, it may be reusedfor medical therapy or other uses. Thus, the liquid activating agent mayitself be a phase change material which not only activates theendothermic or exothermic compound, but is also absorbed or adsorbed bythe silica particles so as to form a PCM/silica gel or dry powdercapable of thermal energy storage upon reuse.

FIGS. 4 and 5 depict a tableware item, a dinner serving tray 30 of thetype used by airlines, etc. that incorporates the present compositiondisposed therein. Tray 30 comprises a plurality of compartments 36a-d toact as receptacles for food and a beverage container. Preferably, tray30 comprises a plastic housing 34 that is filled with composition 38 ofthe present invention which in one embodiment is a water/urea PCM/silicadry powder composition and in another embodiment is a silica based drypowder containing an endothermic or exothermic compound. In thatinstance, a tap 40 may be used for addition of a liquid activatingsolution such as water in order to initiate the endothermic orexothermic reaction. Alternatively, a liquid activating solution such aswater may be contained in cavity 32 separated from composition 38 by arupturable divider (not shown), such as a rupturable membrane, which isruptured to initiate the endothermic or exothermic reaction.

As with the embodiment employing a water/urea phase change material,other uses for an instant cold system, could be for ice cream freezersand other refrigeration devices. With the instant hot system, the devicewould be to heat food or drink. Other uses for an instant hot system ofthis type would be stadium seats, boots, vests, caps, ear muffs,scarves, etc. In order that the invention may be more readilyunderstood, reference is made to the following examples, which areintended to be illustrative of the invention, but are not intended to belimiting in scope.

EXAMPLE 1

This example illustrates the general laboratory procedure for preparinga water/urea clathrate or inclusion compound. Commercial chemical andmedical grades of urea were mixed with water in ratios ranging from90/10% water/urea by weight to 54.5/45.5% by weight. All of the samplesexcept for the 54.5/45.5% ratio dissolved quickly at room temperature.This sample required heating to obtain a solution that did not appear tobe complete when the sample cooled down to room temperature.

Analysis of thermal energy storage characteristics of the solutions wasperformed. The compositions containing water/urea in ratios of 30/1 downto 13/1 showed two distinct melting and freezing temperatures, one ofwhich was essentially water, and the second apparently a clathrate orinclusion compound with a melting temperature of about -11° C. and afreezing temperature of about -15° C.

As the water content in the composition was decreased to water/urea 13/1and lower, the melting and freezing attributed to a separate water phasedisappeared altogether, and a single clathrate melting and freezing near-11° C. and -15° C. respectively remained. Further, the thermal energystorage characteristics of the water/urea phase change materials wereabout 70 cal/gm, which is close to the accepted value of 80 cal/gmattributed to water. Comparisons were made with water modified withethanol at compositions of water/ethanol of 48/1 to 12/1. Suchcompositions exhibited normal freezing point depression and aprogressively lower heat of fusion as more ethanol was added to bringthe molar composition to water/ethanol 12/1.

The data regarding thermal energy storage characteristics is summarizedin Tables 1, 2 and 3 wherein the differential scanning calorimetry (DSC)data includes the melting temperature in degrees centigrade (Tm° C.),the freezing temperature in degrees centigrade (Tc°C), the differencebetween melting and freezing temperature (Tm-Tc° C.), the heat of fusionin calories per gram (ΔHf Cal/gm) and the heat of crystallization incalories per gram (ΔHc Cal/gm).

                  TABLE 1                                                         ______________________________________                                                                  Tm-Tc ΔHf                                                                             ΔHc                             Material Tm °C.                                                                          Tc °C.                                                                         °C.                                                                          Cal/gm  Cal/gm                                ______________________________________                                        H.sub.2 O/EtOH                                                                         -2.8     -14.2   16.2  38.8    40.1                                  95/5 wt.                                                                      48.8/1 molar                                                                  H.sub.2 O/EtOH                                                                         -5.7     -15.3   9.6   30.6    31.5                                  90/10 wt.                                                                     23/1 molar                                                                    H.sub.2 O/EtOH                                                                         -7.4     -16.1   8.7   27.3    27.8                                  85/15 wt.                                                                     14.5/1 molar                                                                  H.sub.2 O/EtOH                                                                         -10.8    -21.6   10.8  22.1    22.0                                  82.5/17.5                                                                     12/1 molar                                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                  Tm-Tc ΔHf                                                                             ΔHc                             Material Tm °C.                                                                          Tc °C.                                                                         °C.                                                                          Cal/gm  Cal/gm                                ______________________________________                                        Water/Urea                                                                             -11.04   -20.80  9.76  68.73   62.86                                 90/10 wt.                                                                               -3.37   -14.00  10.72                                               30/1 molar                                                                    Water/Urea                                                                             -11.33   -23.49  12.15 58.39   51.92                                 85/15 wt.                                                                               -6.99   -11.51  9.64                                                17.7/1 molar                                                                  Water/Urea                                                                             -11.01   -16.54  5.62  62.79   59.76                                 80/20 wt.         -14.73  3.32                                                13.3/1 molar                                                                  ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                  Tm-Tc ΔHf                                                                             ΔHc                             Material Tm °C.                                                                          Tc °C.                                                                         °C.                                                                          Cal/gm  Cal/gm                                ______________________________________                                        Water/Urea                                                                             -10.7     -14.8  4.1   72.5    69.0                                  78/20 wt.                                                                     13.0/1 molar                                                                  Water/Urea                                                                             -10.8     -15.7  4.9   71.7    68.6                                  85/15 wt.          -16.9                                                      17.7/1 molar                                                                  Water/Urea                                                                              -10.71  -15.83  5.12  72.9    69.3                                  80/20 wt.         -17.14  6.43                                                13.3/1 molar                                                                  ______________________________________                                    

EXAMPLE 2

Several samples of water/urea phase change material/hydrophilic silica(Cabot MS-7), i.e. PCM/silica dry powder, were prepared using thegeneral mixing procedure described in related U.S. Pat. No. 5,282,994.The composition of the clathrate portion was varied to includewater/urea ratios of 13.3/1, 13.0/1, 12.5/1 and 12.0/1. Free-flowing drypowders were formed in each case at PCM/silica compositions of 60/40% byweight. Thermal energy storage was analyzed by differential scanningcalorimetry, and the data is shown in Table 4 in the same manner as thedata, shown in Tables 1, 2 and 3. It was observed that all of thesamples showed an undesirable low freezing temperature of about -25° C.,versus about -15° C. for the 100% clathrate of the same composition.Additionally, the thermal energy storage was at a significantly lowervalue of 30 cal/gram, whereas the storage predicted from the value ofthe neat water/urea phase change material, multiplied by the percentageof phase change material in the PCM/silica dry powder should have beenabout 40 cal/gram. It was concluded that the hydrophilic fumed silicawas bonding with some of the water and thereby destroying thestoichiometry of the water/urea phase change material.

                  TABLE 4                                                         ______________________________________                                                                   Tm-Tc ΔHf                                                                            ΔHc                             Material   Tm °C.                                                                         Tc °C.                                                                         °C.                                                                          Cal/gm Cal/gm                                ______________________________________                                        Water/Urea -13.7   -25.4   11.7  29.2   28.8                                  PCM                                                                           80/20 Wt.                                                                     PCM/Silica 60/40                                                              Water/Urea -13.6   -28.2   13.6  26.8   26.2                                  PCM                                                                           78/20                                                                         PCM/Clathrate                                                                 60/40                                                                         Water/Urea -13.2   -25.7   12.5  29.7   29.4                                  PCM 75/20                                                                     PCM/Silica 60/40                                                              Water/Urea -13.4   -27.2   13.8  30.6   30.4                                  PCM 72/20                                                                     PCM/Silica 60/40                                                              ______________________________________                                    

EXAMPLE 3

Using the general mixing procedure described in U.S. Patent No.5,282,994, a sample of water/urea phase change material (10/1 molar) wasmixed with PPG BXS 318 from PPG Industries of Pittsburgh, Penn.(BXS-318) hydrophobic silica (treated with 1 pph silane coupling agent)to form a PCM/silica dry powder. The silica was added to the liquidwater/urea phase change material at room temperature, and a free flowingdry powder was obtained at a composition of water/urea/BXS 318 of 67/33%by weight. Thermal energy storage was determined by differentialscanning calorimetry, at a heating and cooling range of 2° C./minute. Incontrast to the sample of similar water/urea/silica dry powder made withCabot MS-7, the PCM/silica dry powder made with the PPG BXS 318 had ahigher freezing temperature such as would be within the capacity of theordinary freezers, or the freezer compartment on a householdrefrigerator. Specific data computed from the DSC analysis were:

                  TABLE 5                                                         ______________________________________                                                                   Tm-Tc ΔHf                                                                            ΔHc                             Material   Tm °C.                                                                         Tc °C.                                                                         °C.                                                                          Cal/gm Cal/gm                                ______________________________________                                        Water/Urea -12.61  -17.52  4.91  39.02  37.56                                 (10/1) BXS-318                                                                Water/Urea -12.45  -17.38  4.91  38.04  36.64                                 (67.8/32.2)                                                                   ______________________________________                                    

EXAMPLE 4

Instant cold packs, both of the prior art type, and of the typedemonstrating the present invention were prepared as set forth in Table6. Those prepared in accordance with the present invention, i.e.utilizing a silica based gel or dry powder, were prepared usinghydrophilic precipitated silica particles from PPG Industries ofPittsburgh, Penn. (referred to as Silica ABS or ABS Silica) having asurface area of 150 m² /gram and an ultimate particle size of about0.022 microns.

In Table 6, samples 17 and 18 show the prior art form of cold packsbased on ammonium nitrate and water. Sample 18 contains seven parts ofsodium chloride as a freezing point depressant. If salt were to be used,the concentration would need to be further increased (as in thecommercial products) to prevent freezing to a hard lump of ice in reuse.Samples 19 and 20 both contain ABS silica sufficient to form a drypowder. Sample 20 dry powder contains additionally seven parts of NaClfreezing point depressant which is, in some respects, actuallyundesirable since it will make recharging by freezing, and reuse as aphase change material difficult. Accordingly, the composition of sample19 without salt is preferred. Samples 21 and 22 of Table 6 both containsufficient ABS silica to form a gel and the water used contains urea inthe form of a water/urea (75/25) solution. This addition effectivelyprevents the gel from freezing to a hard ice at ordinary freezertemperatures and thus, provides reuse capability. An improved gelformulation is contained as Sample 26. In this case, the urea freezingpoint depressant is mixed directly into the ammonium nitrate prior toaddition of the activating water. Since the urea adds furtherendothermic heat, this formulation may be preferred since it will go tolower temperatures and does not require any premixing of the urea withwater (e.g. samples 21 and 22). The minimum temperature of Sample 26 isabout 20° F. The minimum temperature for the sample 21 without the ureaadded to the dry powder is about 28° F.

Accordingly, this example shows endothermic compositions comprisingammonium nitrate, urea, or combinations of ammonium nitrate and ureawith and without a freezing point depressant (salt or urea) and waterplus sufficient hydrophilic (fumed or precipitated) silica to form areusable gel (at lower silica concentrations) or a reusable dry powder(at higher concentrations) when activated with water.

                  TABLE 6                                                         ______________________________________                                        INSTANT COLD PACKS BASED ON AMMONIUM                                          NITRATE/WATER FOR PHASE                                                       CHANGE LABORATORIES                                                                                 Dry    Dry                                                        One  One    Pow-   Pow-                                                       Use  Use    der    der  Gel  Gel  Gel                                         17   18     19     20   21   22   26                                ______________________________________                                        Water (gms) 140    140    140  140  --   --   140                             Water/Urea (75/25)                                                                        --     --     --   --   187  187  --                              gms.                                                                          Urea gms.   --     --     --   --   --   --   47                              Silica ABS gms.                                                                           --     --     73.4 73.4 70   73.4 70                              Water/Silica %                                                                            --     --     65/  65/  72.7/                                                                              71.9/                                                                              72/                                                       35   35   27.3 28.1 28                              NH.sub.4 NO.sub.3 gms.                                                                    168.sup.(1)                                                                          168.sup.(1)                                                                          168.sup.(2)                                                                        168.sup.(2)                                                                        168.sup.(2)                                                                        168.sup.(2)                                                                        168.sup.(2)                     Salt (gms)  --     7      --   7    --   --   --                              ______________________________________                                         .sup.(1) Ammonium Nitrate Prills (Pellets)                                    .sup.(2) Ammonium Nitrate Powder                                         

EXAMPLE 5

Instant hot packs demonstrating the present invention were made as setforth in Table 7. The Silica ABS is the same as that discussed inExample 4.

The two hot packs include duplicates of a gel (No. 21) and dry powder(No. 23) respectively based on calcium chloride and ABS silica withwater to activate. The gels give a higher maximum temperature, but thedry powders have better conformability. Maximum temperatures in therange of 155° to 145° F. were recorded for the gel and dry powderrespectively.

Accordingly, this example shows a chemical composition for "instant hot"applications comprised of a high exothermic molal heat of solutioncompound having a relatively low molecular weight, a high solubility inwater, plus hydrophilic silica in an amount sufficient to form a gel ora dry powder when subsequently mixed with separately contained water inratios determined by the solubility of the exothermic compound in water.

                  TABLE 7                                                         ______________________________________                                        INSTANT HOT PACK BASED ON CALCIUM                                             CHLORIDE/WATER/SILICA                                                         Sample Number     (21)     (23)                                               Sample Type       Hot Pack Hot Pack                                           (Gel or Dry Powder)                                                                             Gel      Dry Powder                                         ______________________________________                                        1. Water, distilled (gms)                                                                       200      200                                                2. Ammonium Nitrate (gms)                                                     3. Calcium Chloride (gms)                                                                       119      119                                                4. Silica ABS (gms)                                                                              53      98.5                                               5. Water/Silica (%)                                                                             79/21    67/33                                              6. Min. or Max. Temp. °F.                                                                155      147                                                ______________________________________                                    

The primary advantage is that the instant cold or instant hot reactioncan be initiated by mixing with water anytime, anyplace, and the needfor "charging" by prefreezing the instant cold or microwave heating theinstant hot system is eliminated altogether. The primary disadvantage ofthe instant hot and instant cold gels and dry powders is that theycannot supply cold or heat for as long a time period as the comparablephase change materials, and the instant cold or hot is provided on athermocline of ascending or descending temperature--not on a plateau ofconstant temperature as in the comparable phase change materials.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. An article for use in thermal energy generationcomprising a container having a plastic housing, a mixture ofhydrophilic silica particles and an endothermic or exothermic compounddisposed in the plastic housing of said container, and a liquidactivating solution disposed in the plastic housing of said container,separated from said mixture by a rupturable device.
 2. The article ofclaim 1 wherein said hydrophilic silica particles are precipitatedsilica particles.
 3. The article of claim 2 wherein said precipitatedsilica particles have a surface area of 150 m² /gram and an ultimateparticle size of about 0.022 microns.
 4. The article of claim 2 whereinsaid compound is an endothermic compound selected from the groupconsisting of ammonium nitrate, urea, and mixtures thereof.
 5. Thearticle of claim 2 wherein said compound is an exothermic compoundselected from the group consisting of calcium oxide and calciumchloride.
 6. The article of claim 1 wherein said silica particles arepresent in an amount by weight of the total article of 15 to 30% to forma gel.
 7. The article of claim 1 wherein said silica particles arepresent in an amount by weight of the total composition of 25 to 40%, toform a dry powder.
 8. The article of claim 1 wherein said liquidactivating solution is water.
 9. The article of claim 1 wherein saidsilica particles have been surface treated with 0.5-5 parts per hundredby weight of a silicone resin or a silane compiling agent to render themhydrophobic.